An assembly of the aforesaid type for testing bank notes with magnetic properties, for example magnetic particles, is known from EP-A2 0 295 229. The measuring apparatus comprises a permanent magnet and a magnetic circuit which is located above the permanent magnet. The magnetic circuit has two like-shaped legs made of the same material with an air gap between the legs and a coil wound about the yoke of the magnetic circuit. The permanent magnet is positioned relative to the magnetic circuit in such a way that its field extends in the air gap in a plane perpendicular to the direction of transport. The bank note is moved past the air gap in such a way that the static magnetic field acts on the magnetic particles. The magnetized particles moved past the air gap induce in the magnetic circuit a change of flux which produces a corresponding signal at the coil.
The essence of the known apparatus is that the static permanent field is oriented such that its lines of flux are not coupled into the magnetic circuit, which is intended to avoid a saturation magnetization of this circuit.
However, the selected arrangement of the magnetic circuit and the permanent magnet has the disadvantage that only a small part of the total static magnetic field can act on the magnetic particles. This makes the degree of magnetization of these particles accordingly small, so that the signals induced in the coil are also small.